How is compressed air flow measured?

Why you need to measure compressed air

Compressed air is an important secondary energy source for enterprises, factories and institutions, most of which are converted from electrical energy or thermal energy through compressors. When the air pressure value is lower, compressed air is generated by the blower.
In the production process of chemical industry, there is an important process - oxidation reaction, which uses air as a raw material, and conducts a chemical reaction with another raw material under specified conditions. Air mass flow that is too large or too small can have a critical impact on safe production, product quality and the consumption of valuable raw materials. In this case, the air flow measurement accuracy is particularly high, often with automatic adjustment.
The combustion process in boilers and various industrial kilns is also an oxidation reaction in essence. The measurement of combustion-supporting air flow is very important for environmental protection and economical combustion.

Features of Compressed Air Flow Measurement

① Compressed air flow measurement pipeline has large vibration.
Vibration is a concern for air flow meters installed in both the compression plant and the blower room. This vibration mainly comes from the compressor and blower, and the vibration of the machine can be transmitted to a long distance through the air duct. Among them, the reciprocating compressor has the largest vibration. The vibration generated by the operation of the large reciprocating compressor often drives the workshop and the surrounding ground to vibrate together, posing a threat to the accurate and reliable operation of the relevant air flow meter. It causes the pivot point of the lever differential pressure transmitter to move and cause the instrument to drift. Vibration causes the vortex flow sensor to generate an interference signal corresponding to the vibration frequency, causing the flow indication value to be significantly higher.
② Compressed air contained with water.
Compressed air is taken from the atmosphere, which always contains a certain amount of water vapor. For larger size orifice flowmeters used to measure the flow of compressed air, there is often accumulated water in front of the orifice, which will affect the measurement accuracy. There is often a section of water in the impulse line, which causes the differential pressure measured by the differential pressure transmitter to be inconsistent with the differential pressure generated by the throttling device. These are common causes of errors caused by water in the air. In addition, due to the high content of nitrogen oxides in the urban atmosphere, the water droplets in the compressed air are acidic, causing the surface of the orifice ring chamber to corrode and the inner wall of the pipeline to corrode, making the surface rough. When the iron oxide produced by corrosion becomes dry under certain conditions, it is easy to fall off the inner wall of the pipe and be brought to the front of the orifice by the airflow, which will also affect the flow indication value.
③ Pulsating flow from the compressed air flow measurement pipeline.
Most compressor and blower outlet fluids contain a certain amount of pulsation. The flow pulsation causes the differential pressure flowmeter, vortex flowmeter and other flowmeters to show high values, which causes the float in the variable area flowmeter to jump up and down.

Compressed Air Flow Meter types

There are many kinds of measurement instruments that can be used to measure air flow. The most used types are variable area flowmeters, throttling differential pressure flowmeters, vortex flowmeters and uniform velocity tube flowmeters and thermal mass flow meters.

Variable area flowmeter to measure compressed air

Variable area flow meters, which also called rotameters are widely used in medium and small labs or not large pipelines ,such as 1/2”, 3/4 in,1 inch, 2 inch ,3 “or 4”.,etc. because they are simple and cheap price air flow meter, and are suitable for general indications without any power supply . There are two types of rotameters: glass tube type and metal tube rotameter. The disadvantage of glass VA flow meter is that the pressure resistance is not high and the glass cone tube is fragile. In addition, the fluid temperature and pressure have a great influence on the indication value. Generally, it can be manually converted according to the actual temperature and pressure of the fluid. Rotameter now option with transmitters to have remote signal 4-20mA for various process control purpose.

Differential Pressure flow meter to measure compressed air flow

DP flow meters have a long history in air flow measurement, such as orifice plate flow meter,V-cone flow meters.,etc. Although the throttling differential pressure flowmeter has major shortcomings such as narrow range, troublesome installation and maintenance, and large pressure loss, it still has high reliability, good stability and anti-interference ability in compressor and blower with obvious vibration.
The most important thing in measuring air flow with a throttle differential pressure flowmeter is to deal with the problems of water accumulation before the throttling part, water accumulation in the high and low pressure chambers of the DP transmitter, and water accumulation in the pressure piping.

Vortex flowmeter to measure compressed air

In places where there is no vibration work or obvious vibration, the significant advantages of using a vortex flowmeter to measure air flow are small pressure loss, high accuracy, wide range, and small maintenance workload. The piezoelectric vortex flowmeter can withstand 0.2g vibration. Under normal pressure conditions, the lower limit of the measurable flow velocity is 6m/s. Capacitive vortex flowmeter can withstand (0.5~1)g vibration. Under normal pressure conditions, the lower limit of the measurable flow velocity is 4m/s. Therefore, both types of vortex flowmeters are not suitable for places with large vibrations. In recent years, some companies have introduced digital signal processing (DSP) technology in the manufacture of vortex flowmeters, so that the anti-vibration performance of the instrument has been greatly improved, and the lower limit of the measurable flow rate has also been significantly improved.
Features of Vortex flow meter to measure compressed air
Industrial in-line vortex flow meter with various size options: 1 inch compressed air flow meter, 2in dial size, 3 inch ,4 inch, 6 inch , 8 inch, 10 inch air flow sensors;
Option with insertion vortex flow meters for compressed air flow measurement;
Digital display to show compressed air flow rate and total flow;
Operation with built-in temperature sensor and pressure sensor to have air flow information at standard condition, flow rate unit: Nm3/h;
Different process connection: wafer, flange or thread connection;

Differential pressure equalizing tube flowmeter

The uniform velocity tube flowmeter has its unique advantages for the measurement of large-diameter air flow. It is cheap, simple and reliable, and easy to install and maintain.

Thermal mass flow meter for compressed air mass flow measurement

• Thermal mass flowmeter is based on the principle of the relationship between the energy required to increase the temperature of the air and the mass flow of the air when heating the air , or by measuring the heat transfer and heat transfer when heating the fluid to measure the fluid mass.
• Thermal mass flow meter can be made into insertion type mass flow sensor which is suitable for large pipeline air flow measurement, such as 8 inch dial size, 10inch, 12 inch, 14 inch , 20 inch, 36inch .etc.
• Insertion thermal mass flow meter has low price starts from $ 800.
• Direct air mass flow measurement without any temperature and pressure correction.
• Standard with MODBUS RTU communications;